1. Field of the Invention
The present invention relates to the field of applied ignition internal combustion engine. Specifically, the present invention relates to at least one cylinder in which a piston slides along the cylinder axis, with a cylinder head Capping the cylinder and, together with the piston and cylinder, defining a combustion chamber, the piston having a recess open toward the cylinder head. Engines of the aforementioned type that operate on a lean mixture are particularly the subject of the invention.
2. Description of the Prior Art
Advantageously, applied ignition engines operating on a highly dilute mixture (richness of the mixture less than 0.6 for example) considerably reduce pollutant emissions such as nitrogen oxides (NO.sub.X).
However, this technology may raise problems linked to flame initiation and propagation which result in combustion instabilities and increased levels of unburned hydrocarbons.
An engine manufacturer familiar with this type of engine must thus pay special attention to the quality of combustion which must be rapid and stable to reduce NO.sub.X emissions while increasing the energy efficiency of the engine.
Combustion characteristics depend on geometric factors (shape of chamber, shape of recess in piston, position of spark plug, and position of intake system) and aerodynamic factors (turbulence and speed), distribution factors, etc.
Optimization of the internal aerodynamics proves to be an important step in the design of an applied ignition engine operating on a lean mixture. The design of the internal aerodynamics in the chamber at the time of ignition must contribute to increasing combustion rates and favoring stability of the flame upon ignition. The flow characteristics at this time in the chamber depend on the result of the interaction between the post-intake aerodynamics and the shape of the combustion chamber. The turbulence levels and rates must in particular be optimized to favor ignition stability and flame propagation in the chamber.
Various geometries have been recently developed, particularly of the recess formed in the piston itself
Thus, U.S. Pat. No. 5,351,665 describes a combustion chamber formed in the piston by a recess with a particular geometry.
Document French Patent 2 713 282 discloses another geometry of the recess formed in the upper part of the piston.